JP3307236B2 - Catalyst deterioration determination device for internal combustion engine - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for judging deterioration of a catalyst for purifying exhaust gas of an internal combustion engine (three-way catalyst), and in particular, deterioration of judgment accuracy caused by fluctuation of an oxygen balance point in the three-way catalyst. TECHNICAL FIELD The present invention relates to a catalyst deterioration determination device for an internal combustion engine that can prevent the occurrence of deterioration.

[0002]

2. Description of the Related Art Conventionally, in internal combustion engines for automobiles, oxidation of unburned components (HC, CO) and reduction of nitrogen oxides (NO _x ) in exhaust gas are simultaneously promoted as exhaust gas purification measures. A three-way catalyst is used. In order to increase the oxidation / reduction ability by such a three-way catalyst, it is necessary to control the air-fuel ratio (A / F) indicating the combustion state of the internal combustion engine to be close to the stoichiometric air-fuel ratio (window). for that reason,
In fuel injection control in an internal combustion engine, an O ₂ sensor (oxygen concentration sensor) that detects whether the air-fuel ratio is richer or leaner than the stoichiometric air-fuel ratio based on the residual oxygen concentration in the exhaust gas.
(See FIG. 2), and the air-fuel ratio feedback control for correcting the fuel amount based on the sensor output is performed.

In this air-fuel ratio feedback control, an O ₂ sensor for detecting the oxygen concentration is provided as close to the combustion chamber as possible, that is, upstream of the catalytic converter, but the variation in output characteristics of the O ₂ sensor is compensated. For this reason, a double O ₂ sensor system further provided with a _second O ₂ sensor downstream of the catalytic converter has also been realized. That is, in the downstream side of the catalyst, the exhaust gas is sufficiently stirred, by which is in near equilibrium state by action of the oxygen concentration the three-way catalyst, the output of the downstream O ₂ sensor, the upstream O ₂ sensor Also changes slowly, thus indicating a rich / lean tendency for the entire mixture. The double O ₂ sensor system performs the sub air-fuel ratio feedback control by the downstream O ₂ sensor in addition to the main air-fuel ratio feedback control by the upstream O ₂ sensor, and performs the air-fuel ratio correction coefficient by the main air-fuel ratio feedback control. ,
By correcting based on the output of the downstream O ₂ sensor, variations in the output characteristics of the upstream O ₂ sensor are absorbed,
The aim is to improve the air-fuel ratio control accuracy.

[0004] By performing the precise air-fuel ratio control described above,
Also, catalyst deteriorates due to heat of exhaust gas and poisoning such as lead
Thus, sufficient exhaust gas purification performance cannot be obtained. So
Here, conventionally, various catalyst deterioration detecting devices have been proposed.
I have. One of them is downstream O _TwoAfter warm-up by sensor
O_TwoStorage effect (unburned exhaust holding excess oxygen
Function that is used to purify wastewater)
This is to diagnose catalyst deterioration. In other words, poor catalyst
Causes a reduction in purification performance after warm-up
However, this device is_TwoPurification performance to reduce storage effect
Is estimated to decrease and the downstream O_TwoUsing the output signal of the sensor
To find the trajectory length, feedback frequency, etc._Two
Detects deterioration of storage effect and detects catalyst deterioration
Things. For example, Japanese Patent Application Laid-Open No. 5-98948 discloses
The indicated device is under feedback control to stoichiometric air-fuel ratio
At the upstream and downstream O_TwoThe path length of the sensor output
This is a device that detects catalyst deterioration based on these ratios.
You.

On the other hand, in recent years, there has been developed an internal combustion engine that controls an air-fuel ratio so that a three-way catalyst can always exhibit a constant and stable purification performance. That is, the O ₂ storage capacity purifies the exhaust gas by adsorbing excess oxygen when the exhaust gas is lean and releasing insufficient oxygen when the exhaust gas is rich. However, such capabilities are finite. Therefore, in order to utilize the O ₂ storage capacity effectively, the amount of oxygen stored in the catalyst must be adjusted so that the air-fuel ratio of the exhaust gas may be either rich or lean next. Predetermined amount (for example, half of the maximum oxygen storage amount)
It is important to maintain a constant O ₂ adsorption / desorption effect, and as a result, a constant oxidation / reduction capacity of the catalyst is always obtained.

In order to maintain the purification performance of the catalyst as described above, O
_{(2) In} an internal combustion engine that controls the storage amount constant,
An A / F sensor (see FIG. 3) capable of linearly detecting an air-fuel ratio is used as an upstream air-fuel ratio sensor, and feedback control (F / B control) by proportional and integral operations (PI operations) is performed. In other words, the next time fuel correction amount = K _p * (difference this time of fuel) + K _s * Σ
(Fuel difference up to now) Fuel difference = (Actual amount of fuel actually burned in the cylinder)-(Target in-cylinder fuel amount with stoichiometric intake air) Fuel amount actually burned in the cylinder = Air flow detection value /
The air-fuel ratio detection value K _p = proportional term gain K _s = integral term gain By the calculation, the feedback fuel correction amount is calculated.

As can be seen from the above equation for calculating the fuel correction amount, the proportional term is a component acting to maintain the air-fuel ratio at the stoichiometric air-fuel ratio, as in the feedback control by the O ₂ sensor. It is a component that acts to eliminate the deviation (offset). That is, the action of the integral term results in that the O ₂ storage amount in the catalyst is kept constant. For example, when lean gas is generated due to sudden acceleration or the like, the action of the integral term generates rich gas, and the effect of lean gas generation is offset.

In such an O ₂ storage amount constant control system, an O ₂ sensor may be provided downstream of the catalyst in order to compensate for variations in the output characteristics of the upstream A / F sensor. Therefore, also in this case, similarly to the double O ₂ sensor system, it is conceivable to detect deterioration of the catalyst by detecting a decrease in the O ₂ storage effect of the catalyst with the O ₂ sensor.

However, while the upstream A / F sensor outputs a signal proportional to the residual oxygen concentration in the exhaust gas, the O ₂ sensor used as the downstream air-fuel ratio sensor detects the residual oxygen concentration in the exhaust gas. Since it has the corresponding Z-type output characteristic, when performing the deterioration determination based on the trajectory length ratio, there is a possibility that an erroneous determination may occur due to a difference in the output characteristic. That is, when the amplitude of the residual oxygen concentration of the gas (catalyst-containing gas) flowing into the three-way catalyst is extremely small or extremely large, regardless of the degree of deterioration of the three-way catalyst, the trajectory length of the A / F sensor output and O ₂ The ratio of the sensor output to the track length is almost "
1.0 ", the degree of deterioration cannot be accurately determined.

[0010] In order to solve this problem, the present applicant has performed an output conversion using a function having substantially zero output in a minimum region of the output of the upstream A / F sensor and having a saturation characteristic in a maximum region of the upstream A / F sensor. A catalyst deterioration determination device for an internal combustion engine that suppresses the occurrence of erroneous determination in the minimum and maximum regions of the / F sensor output has already been proposed (Japanese Patent Application No. Hei 8-60).
No. 933).

[0011]

However, even if the three-way catalyst is normal, if the oxygen balance point is different, that is, if the balance between the oxygen adsorption capacity and the oxygen release capacity is different, the fluctuation range of the air-fuel ratio of the gas containing the catalyst is changed. Even if the _two values are the same, there is a possibility that the accuracy of determining the deterioration of the three-way catalyst may be deteriorated due to the difference in the swing (amplitude) of the O ₂ sensor.

FIG. 4 is an explanatory view of the above problem.
Represents the output of the upstream A / F sensor, and (b) represents the output of the O ₂ sensor. That O ₂ sensor time t ₃ before is rich, shows the case of inverted from rich to lean at time t _3. Air-fuel ratio swings largely lean side when the O ₂ sensor output catalysts containing gas at time t ₁ since largely fluctuates from the rich state to the lean direction, the trajectory of the O ₂ sensor downstream oxygen sensor output becomes longer.

On the other hand, at time t_TwoGas containing catalyst
If the air-fuel ratio of the fuel_TwoSensor output
Swings from the rich state to the rich direction, so the output
Is saturated and O_TwoThe trajectory of the oxygen sensor output downstream of the sensor is short
Become. Similarly, time t_FourThe air-fuel ratio of the gas containing the catalyst
It is O when it swings to the lean side greatly _TwoSensor output is lean
, The output saturates and O _Two
The locus of the sensor output becomes shorter.

[0014] In contrast, since the air-fuel ratio of the catalyst entering gas swings largely rich side O ₂ sensor output is largely fluctuates from the rich state to the lean direction at time t _5, O ₂
The locus of the sensor output becomes longer. That is, even if the fluctuation width of the air-fuel ratio of the gas containing the catalyst is the same, the amplitude of the O ₂ sensor output depends on whether the exhaust gas (catalyst exit gas) flowing out of the three-way catalyst is in a lean state or a rich state. Different,
There is a possibility that the accuracy of the deterioration determination is deteriorated.

The present invention has been made in view of the above problems, and is caused by a change in the oxygen balance point in the three-way catalyst.
An object of the present invention is to provide a catalyst deterioration determination device for an internal combustion engine that can prevent deterioration of the determination accuracy.

[0016]

FIG. 1 is a basic block diagram of an apparatus for judging catalyst deterioration of an internal combustion engine according to the present invention. According to a first aspect of the present invention, there is provided a catalyst deterioration determination device for an internal combustion engine, which is provided upstream of an exhaust purification catalyst provided in an exhaust passage of the internal combustion engine and has an output characteristic that is substantially proportional to an oxygen concentration in exhaust gas. Sensor 11 and air-fuel ratio feedback control means 13 for performing feedback control according to the output of upstream air-fuel ratio sensor 11 so that the engine air-fuel ratio becomes the target air-fuel ratio.
And a downstream air-fuel ratio sensor 12 provided on the downstream side of the exhaust purification catalyst and having an output characteristic corresponding to the oxygen concentration in the exhaust gas.
Air-fuel ratio center value estimating means 1 for estimating the air-fuel ratio center value of the exhaust purification catalyst based on the output of the downstream air-fuel ratio sensor 12
4 and the upstream air-fuel ratio sensor 11 according to the air-fuel ratio center value in the exhaust purification catalyst estimated by the air-fuel ratio center value estimating means 14.
Upstream air-fuel ratio sensor output converting means 15 for converting the output of the air-fuel ratio into an output for deterioration determination, and air-fuel ratio feedback control means 1
3. The deterioration of the exhaust gas purification catalyst is determined based on the deterioration determination output converted by the upstream air-fuel ratio sensor output conversion means 15 and the output of the downstream air-fuel ratio sensor 12 within a predetermined period during the execution of the air-fuel ratio feedback control by the control unit 3. And a catalyst deterioration determining means 16.

According to the present device, the conversion function for converting the output of the upstream air-fuel ratio sensor to the output for determining deterioration is changed according to the air-fuel ratio center value in the exhaust purification catalyst. According to a second aspect of the present invention, the air-fuel ratio center value estimating means calculates a temporal moving average value of the output of the downstream air-fuel ratio sensor. According to the present device, the conversion function for converting the output of the upstream air-fuel ratio sensor into the output for deterioration determination is changed according to the temporal moving average value of the output of the downstream air-fuel ratio sensor.

According to a third aspect of the present invention, the upstream air-fuel ratio sensor output conversion means includes an upstream air-fuel ratio sensor.
When the amplitude of the output of the fuel ratio sensor increases, the output for deterioration determination
Of the upstream air-fuel ratio sensor using the conversion characteristic
The force is converted into a deterioration determination output. By this device
If the amplitude of the upstream air-fuel ratio sensor is large,
A conversion characteristic that saturates the separate output is used. Claim 4
Such a device for determining catalyst deterioration of an internal combustion engine includes an upstream air-fuel ratio sensor.
The sensor output conversion means outputs the air-fuel ratio center value
It is determined that the air-fuel ratio center value of the gas purification catalyst is on the rich side.
If the output of the upstream air-fuel ratio sensor is rich
Characteristics to correct the deterioration determination output to the stoichiometric air-fuel ratio side
The output of the upstream air-fuel ratio sensor using the
And a rich-side conversion means for converting into With this device
If the air-fuel ratio of the catalyst is rich, the
In the case of h, it is corrected to the stoichiometric air-fuel ratio side. Claim 5
The internal combustion engine catalyst degradation determination device is an upstream air-fuel ratio sensor.
The output conversion means uses the air-fuel ratio
If it is determined that the center value of the air-fuel ratio of the
When the output of the upstream air-fuel ratio sensor is lean
The conversion characteristic that corrects the deterioration determination output to the stoichiometric air-fuel ratio side is used.
To convert the output of the upstream air-fuel ratio sensor to the output for
And a lean-side conversion means for calculating. According to this device,
If the catalyst has a lean air-fuel ratio, the upstream air-fuel ratio
At this time, it is corrected to the stoichiometric air-fuel ratio. According to claim 6
The catalyst deterioration determination device of the fuel engine outputs the upstream air-fuel ratio sensor.
The conversion means is a means for estimating the exhaust gas in the air-fuel ratio center value estimation means.
When it is determined that the air-fuel ratio center value of the medium is on the rich side
Deteriorates when the output of the upstream air-fuel ratio sensor is rich
Use the conversion characteristic to correct the discrimination output to the stoichiometric air-fuel ratio side.
The output of the upstream air-fuel ratio sensor to the output for deterioration determination
Rich-side conversion means and air-fuel ratio center value estimation means
It is determined that the center value of the air-fuel ratio of the exhaust purification catalyst is on the lean side.
The output of the upstream air-fuel ratio sensor is lean
Conversion characteristic to correct the deterioration determination output to the stoichiometric air-fuel ratio
The output of the upstream air-fuel ratio sensor for deterioration determination using
Comprising a lean-side converting unit that translated into force. By this device
If the air-fuel ratio of the catalyst is rich, the air-fuel ratio
Is switched to the stoichiometric air-fuel ratio when the
If lean, stoichiometric when upstream air-fuel ratio is lean
Corrected to the ratio side.

According to a seventh aspect of the present invention, there is provided a method for judging catalyst deterioration of an internal combustion engine.
In the device, the downstream air-fuel ratio sensor is an oxygen
In the exhaust gas purification catalyst estimated by the fuel ratio center value estimation means 14,
Output to the upstream air-fuel ratio sensor 11 based on the air-fuel ratio center value
Upper and lower limit value determining means 17 for determining upper and lower limit values for
The output of the upstream air-fuel ratio sensor 11 is determined by the upper / lower limit value determining means 14.
When the value exceeds the specified upper and lower limits,
Deterioration determination stop means 1 for stopping the deterioration determination of the exhaust purification catalyst
8 is further provided. According to this device, the upstream air-fuel ratio
When the sensor output exceeds the upper and lower limits, the exhaust purification catalyst
The discrimination is stopped. A catalyst for an internal combustion engine according to claim 8.
In the deterioration determination device, the upper / lower limit value determining means includes an upstream air-fuel ratio sensor.
The deterioration determination output converted by the sensor output conversion means
But the output of the upstream air-fuel ratio sensor begins to saturate
Values are upper and lower limits. According to this device, the converted deterioration
The output for determination does not saturate, but the output of the upstream air-fuel ratio sensor
Are determined as upper and lower limits.

[0020]

Embodiments of the present invention will be described below with reference to the accompanying drawings. FIG. 5 is an overall schematic diagram of an electronically controlled internal combustion engine including the catalyst deterioration determination device according to one embodiment of the present invention. Air required for combustion of the internal combustion engine is filtered by an air cleaner 2, passed through a throttle body 4, and distributed to an intake pipe 7 of each cylinder by a surge tank (intake manifold) 6. The intake air flow rate is adjusted by a throttle valve 5 provided on the throttle body 4. The intake air temperature is detected by an intake air temperature sensor 43. Further, the intake pipe pressure is detected by a vacuum sensor 41.

The opening of the throttle valve 5 is detected by a throttle opening sensor 42. When the throttle valve 5 is in the fully closed state, the idle switch 52 is turned on, and the throttle fully closed signal output from the idle switch 52 becomes active. The idle adjustment passage 8 that bypasses the throttle valve 5 is provided with an idle rotation speed control valve (ISCV) 66 for adjusting the air flow during idling.

On the other hand, the fuel stored in the fuel tank 10 is pumped up by the fuel pump 11 and
Is injected into the intake pipe 7 by the fuel injection valve 60. In the intake pipe 7, air and fuel are mixed, and the air-fuel mixture is
It is sucked into the combustion chamber 21 of the internal combustion engine main body, that is, the cylinder 20 via the intake valve 24. In the combustion chamber 21, the air-fuel mixture is compressed by the piston 23, ignited, exploded and burned to generate power. Such an ignition is performed by the igniter 62 receiving the ignition signal,
3 is controlled by energizing and interrupting the primary current, and the secondary current is supplied to the spark plug 65 via the ignition distributor 64.

The ignition distributor 64 includes:
Its axis is, for example, 720 ° in terms of crank angle (CA).
A reference position detection sensor 50 for generating a reference position detection pulse for each CA and a crank angle sensor 51 for generating a position detection pulse for each 30 ° CA are provided. Note that the actual vehicle speed is detected by a vehicle speed sensor 53 that generates an output pulse representing the vehicle speed. The internal combustion engine body (cylinder) 20 is cooled by cooling water guided to a cooling water passage 22, and the temperature of the cooling water is measured by a water temperature sensor 4.
4 detected.

The burned air-fuel mixture is discharged to the exhaust manifold 30 via the exhaust valve 26 as exhaust gas, and then guided to the exhaust pipe 34. The exhaust pipe 34 is provided with an upstream A / F sensor 45 for linearly detecting the air-fuel ratio based on the oxygen concentration in the exhaust gas. Further, a catalytic converter 38 is provided in the exhaust system downstream of the catalytic converter 38. The catalytic converter 38 oxidizes unburned components (HC, CO) in the exhaust gas and reduces nitrogen oxides (NO _x ). And a three-way catalyst which simultaneously promotes the above. The exhaust gas thus purified in the catalytic converter 38 is discharged into the atmosphere.

Further, in this internal combustion engine, the sub-air-fuel ratio feedback control for compensating the variation of the output characteristics of the upstream A / F sensor 45 by changing the control center of the air-fuel ratio feedback control by the upstream A / F sensor 45 is performed. The exhaust system downstream of the catalytic converter 38 is provided with an O ₂ sensor (or a downstream A / F sensor) 46.

Electronic control unit (ECU) 70 for the internal combustion engine
Is a microcomputer system that executes catalyst deterioration determination processing in addition to fuel injection control (air-fuel ratio control), ignition timing control, idle rotation speed control, and the like. The hardware configuration is shown in the block diagram of FIG. . A central processing unit (CPU) 71 according to a program and various maps stored in a read only memory (ROM) 73.
Inputs signals from various sensors and switches via an A / D conversion circuit 75 or an input interface circuit 76, executes arithmetic processing based on the input signals, and drives control circuits 77a to 77d based on the arithmetic results. And outputs control signals for various actuators via the. The random access memory (RAM) 74 is used as a temporary data storage place in the operation / control processing. The backup RAM 79 is provided with a battery (not shown).
Is directly connected to the power supply, and is used to store data (for example, various learning values) to be held even when the ignition switch is off. Each component in the ECU is connected by a system bus 72 including an address bus, a data bus, and a control bus.

An internal combustion engine control process of the ECU 70 executed in the internal combustion engine having the above hardware configuration will be described below. The ignition timing control comprehensively determines the state of the internal combustion engine based on the internal combustion engine rotation speed obtained from the crank angle sensor 51 and signals from other sensors, determines an optimal ignition timing, and determines a drive control circuit 77.
The ignition signal is sent to the igniter 62 via the line b.

In the idle speed control, the idle state is detected by a throttle fully closed signal from an idle switch 52 and a vehicle speed signal from a vehicle speed sensor 53, and is determined by the temperature of the internal combustion engine cooling water from a water temperature sensor 44, and the like. The target rotational speed is compared with the actual internal combustion engine rotational speed, and a control amount is determined so as to be the target rotational speed according to the difference, and the ISC is determined via the drive control circuit 77c.
By controlling the V66 to adjust the amount of air, the optimum idle rotation speed is maintained.

In the following, in order to describe in detail the air-fuel ratio control (fuel injection control) and the catalyst deterioration determination processing according to the present invention, the procedures of related processing routines will be sequentially shown. FIG.
5 is a flowchart showing a processing procedure of a cylinder air amount estimation and a target cylinder fuel amount calculation routine. This routine is
It is executed at every predetermined crank angle. First, the in-cylinder air amount MC _i obtained by the previous execution of this routine
And updating the target cylinder fuel amount FCR _i. That is, MC _i and FCR of the i-th (i = 0, 1,..., N−1) times
_{i is set} to MC _{i + 1} and FCR _{i + 1 of} the “i + 1” th time before (step 702). This is shown in FIG.
In-cylinder air amount MC _i and target in-cylinder fuel amount FC for the past n times
The data of R _i is stored in the RAM 74, and the MC
₀ and FCR ₀ are calculated.

Next, based on the outputs from the vacuum sensor 41, the crank angle sensor 51, and the throttle opening sensor 42, the current intake pipe pressure PM, the internal combustion engine rotational speed NE, and the throttle opening TA are obtained (step 7).
04). Next, the air amount MC ₀ supplied into the cylinder is estimated from the data of PM, NE, and TA (Step 706). In general, the in-cylinder air amount can be estimated from the intake pipe pressure PM and the internal combustion engine rotation speed NE. However, in this embodiment, a transient state is detected from a change in the value of the intake pipe pressure PM. Also, a precise air amount is calculated.

Next, based on the in-cylinder air amount MC ₀ and the stoichiometric air-fuel ratio AFT, an operation of FCR ₀ ← MC ₀ / AFT is executed, and the air-fuel mixture should be supplied into the cylinder to obtain the stoichiometric air-fuel ratio. The target fuel amount FCR ₀ is calculated (step 708). The in-cylinder air amount MC ₀ and the target in-cylinder fuel amount FCR ₀ calculated in this way are the latest data obtained this time, in the form shown in FIG.
Stored in AM74.

FIG. 9 is a flowchart showing the processing procedure of the main air-fuel ratio feedback control routine. This routine is executed every predetermined crank angle. First, it is determined whether a condition for executing feedback is satisfied (step 902). For example, when the cooling water temperature is equal to or lower than a predetermined value, during engine startup, during post-start increase, during warm-up increase, when there is no change in the output signal of the upstream A / F sensor 45,
During fuel cut, etc., the feedback condition is not satisfied,
In other cases, the condition is satisfied. When the condition is not satisfied, the fuel correction amount DF by the feedback control is set to 0 (step 916), and this routine ends.

[0033] During satisfied feedback condition, updates the FD _i (the difference between the actual amount of fuel that is burned in the cylinder and target cylinder fuel amount) fuel amount difference is obtained by the running up to the previous routine . That is, the i-th (i = 0, 1,
.., M−1) times the FD _i before the “i + 1” th times FD _{i
i + 1} is set (step 904). This is to store data of the past m times the fuel amount difference FD _i in RAM 74, calculates a new fuel amount difference FD ₀ time.

Next, the output voltage value VAF of the upstream A / F sensor 45 is detected (step 906). Next, an operation of VAF ← VAF + DV is executed based on the upstream A / F sensor output voltage correction amount DV calculated by the sub air-fuel ratio feedback control described later, and the upstream A / F sensor output voltage VA is calculated.
F is corrected (step 908). By such a correction, the center of the air-fuel ratio fluctuation is gradually shifted until the target voltage is reached in the sub-air-fuel ratio feedback control. Then, by referring to the characteristic diagram of FIG. 3 based on the VAF after such correction, the current air-fuel ratio A
The BF is determined (step 910). The characteristic diagram of FIG. 3 is mapped and stored in the ROM 73 in advance.

Next, the cylinder air amount MC already calculated by the cylinder air amount estimation and target cylinder fuel amount calculation routines.
_n and the target in-cylinder fuel amount FCR _n (see FIG. 8), the calculation of FD ₀ ← MC _n / ABF-FCR _{n is used to} calculate the difference between the fuel amount actually burned in the cylinder and the target in-cylinder fuel amount. The difference is obtained (step 912).
The reason for thus adopting n times before the cylinder air amount MC _n and target cylinder fuel amount FCR _n is now upstream A / F
This is because the time difference between the air-fuel ratio detected by the sensor 45 and the actual combustion is considered. In other words, there is necessary to store the cylinder air amount MC _i and target cylinder fuel amount FCR _i for the past n times are for such a time difference.

Next, the fuel correction amount DF by the proportional / integral control (PI control) is determined by the calculation of DF ← K _fp * FD ₀ + K _fs * ΣFD _i (step 914). In addition,
The first term on the right side is a proportional term of PI control, and _Kfp is a proportional term gain. The second term on the right side is an integral term of PI control, and K _fs is an integral term gain.

FIG. 10 is a flowchart showing the processing procedure of the sub air-fuel ratio feedback control routine. This routine is executed at a predetermined time period longer than that in the main air-fuel ratio feedback control routine. First,
As in the case of the main air-fuel ratio feedback, it is determined whether a condition for executing the sub air-fuel ratio feedback control is satisfied (step 1002). If the condition is not satisfied, the upstream side A / F sensor output voltage correction amount DV is set to 0 (step 1012), and this routine ends.

When the feedback condition is satisfied, this routine
Voltage difference obtained by the previous run of the
O detected_TwoSensor output voltage and target O_TwoSensor output power
Pressure difference) VD_iTo update. That is, the i-th (i =
VD before 0, 1, ..., p-1) times_iBefore the "i + 1" th
VD_{i + 1}(Step 1004). This is the past
Voltage difference VD for p times_iData stored in RAM 74
Then, a new voltage difference VD ₀This is for calculating.

Next, the output voltage VOS of the O ₂ sensor 46
Is detected (step 1006). Then, the VOS
And the target O ₂ sensor output voltage VOST (for example, 0.5
Based on V), the latest voltage difference VD ₀ is obtained by executing the following operation: VD ₀ ← VOS−VOST (step 1008).

[0040] Finally, DV ← by _{K vp * VD 0 + K vs} * ΣVD it becomes operational, determining the upstream A / F sensor output voltage correction amount DV by PI control (step 1010). Note that K _vp and K _vs are the gains of the proportional term and the integral term, respectively. As described above, the correction amount DV thus obtained is used to change the control center voltage of the feedback control by the upstream A / F sensor in the main air-fuel ratio feedback control routine. Note that this routine shows a case where the downstream air-fuel ratio sensor is an O ₂ sensor, but can also be applied to a case where the downstream air-fuel ratio sensor is an A / F sensor (downstream A / F sensor).

FIG. 11 is a flowchart showing the processing procedure of the fuel injection control routine. This routine is executed every predetermined crank angle. First, based on the target in-cylinder fuel amount FCR ₀ calculated in the above-described in-cylinder air amount estimation and target in-cylinder fuel amount calculation routine, and the feedback correction amount DF calculated in the main air-fuel ratio feedback control routine, FI ← The fuel injection amount FI is determined by executing the calculation of FCR ₀ * α + DF + β (step 1102). Note that α and β are a multiplication correction coefficient and an addition correction amount determined by other operation state parameters. For example, α is the intake air temperature sensor 43, the water temperature sensor 4
4 includes a basic correction based on a signal from each sensor, and β includes a correction based on a change in the amount of fuel adhering to the wall surface (which changes with a change in the intake pipe pressure in a transient operation state). include. Finally, the obtained fuel injection amount FI is set in the drive control circuit 77a of the fuel injection valve 60 (step 1104).

FIG. 12 is a flowchart of a deterioration determination main routine executed by the ECU 30, which is executed at predetermined time intervals. Step 120
In 2 to calculate a name better value VOSSM _i of the output VOS of the O ₂ sensor 46 by the following equation. VOSSM _i = VOSSM _i−1 + γ · (VOS−VOS
SM _i-1 ) where γ is the smoothing rate, and VOSSM _i-1 is the smoothing value up to the previous time. In step 1204 determining the deterioration determination output VAFH as a function of the output VAF and O ₂ sensor 46 for a better value VOSSM _i of the upstream A / F sensor 45. This is because the upstream A / F sensor 4
This is a process for ensuring a one-to-one correspondence between the trajectory length of No. 5 and the trajectory length of the O ₂ sensor 46 to prevent erroneous determination.

FIG. 13 shows the output V of the upstream A / F sensor 45.
This is the first conversion map for obtaining the deterioration determination output VAFH from the AF, where the horizontal axis indicates the output VAF of the upstream A / F sensor 45 and the vertical axis indicates the deterioration determination output VAFH. That also O ₂ sensor 46 for a better value VOSSM is a case where the corresponding stoichiometric air-fuel ratio (b), O ₂ O ₂ O ₂ sensor when the deflection of the output is large sensor 46 by the so-called Z characteristic of the sensor 46 The output of 46 saturates.

Therefore, in order to maintain a one-to-one correspondence between the trajectory length of the upstream A / F sensor 45 and the trajectory length of the O ₂ sensor 46, the output is required when the fluctuation of the air-fuel ratio of the gas containing the catalyst is large. Is required to be a conversion function having characteristics as shown by the solid line in (b). However, if this function is used even when the equilibrium point of the three-way catalyst, that is, the oxygen storage amount deviates from the value corresponding to the stoichiometric air-fuel ratio, the output characteristics of the O ₂ sensor 46 may change, which may cause erroneous determination.

Therefore, in the present invention, utilizing the fact that the deviation of the equilibrium point of the three-way catalyst appears as a deviation of the average value VOSSM of the O ₂ sensor 46 from the stoichiometric air-fuel ratio, the average value VOSSM is also used. The conversion function has been changed. That is time when the balance point for the three-way catalyst is shifted to the rich side of the conversion function shown by the solid line in (c), it shifted to the lean side (A)
The conversion function shown by the solid line is used.

The case where the three-way catalyst has not deteriorated has been described above. However, as the three-way catalyst deteriorates, the oxygen storage effect disappears, resulting in a large swing of the O ₂ sensor 46 (a), (b), As shown by the broken line (c), the output is saturated with a small change in the catalyst output gas air-fuel ratio. O ₂ sensor 4
6, the output VAF of the upstream A / F sensor 45 is saturated.
The range in which the conversion value for deterioration determination does not saturate is the deterioration determination execution range.

That is, the smoothed value VOSS of the O ₂ sensor 46
Using M as a parameter, for example, 3 shown in (a) to (c)
The output VAF of the upstream side A / F sensor 45 is stored as a map, and the output VAF of the upstream
Convert to H When the sub-air-fuel ratio feedback control is performed, the upstream A / F sensor output VAF converted into the deterioration determination output VAFH is the one corrected in step 908 of the main air-fuel ratio feedback control routine. When the feedback control is not performed, the upstream A / F sensor output VAF is used as it is.

Next, in step 1206, the upstream A
As a function of the output VAF of the A / F sensor 45 and the smoothing value VOSSM of the O ₂ sensor 46, the upstream A / F sensor 45
Lower limit value TVAFL and upper limit value TVAF of output VAF
Find R. TVAFL = TVAFL (VAF, VOSSM) TVAFR = TVAFR (VAF, VOSSM) Note that the lower limit value TVAFL and the upper limit value TVAFR are obtained as the lean end point and the rich end point of the determination execution range (FIG. 13).

In step 1208, the output VAF of the upstream A / F sensor 45 is set to the lower limit TVAFR and the upper limit TV
It is determined whether it is during the AFL. Upstream A / F sensor 4
5 is lower limit TVAFR and upper limit TVAFL
If it is, the process proceeds to step 1210 to determine that there is no risk of erroneous determination and to perform the deterioration determination. Conversely, the output VAF of the upstream A / F sensor 45 is lower than the lower limit TVAF.
If it is not between R and the upper limit TVAFL, this routine is ended directly to avoid erroneous determination.

In step 1210, it is determined whether the monitoring condition for determining the deterioration of the three-way catalyst is satisfied. The deterioration of the three-way catalyst is determined when all of the following conditions are satisfied. (1) The air-fuel ratio feedback control by the upstream A / F sensor 45 is being executed. (2) The air-fuel ratio feedback control by the O ₂ sensor 46 is being executed. (3) The internal combustion engine load is equal to or more than a predetermined value.

Therefore, when any of the above conditions (1) to (3) is not satisfied, this routine is directly terminated without performing the deterioration judgment. Conversely, if all of the above conditions (1) to (3) are satisfied, the trajectory length calculation processing is executed in step 1212, and the deterioration determination processing is executed in step 1214, and this routine ends. The details of the trajectory length calculation processing and the deterioration determination processing will be described later.

FIG. 14 is a flowchart of the trajectory length calculation processing executed in step 1212. The trajectory length LVAFH of the output of the upstream A / F sensor 45 is obtained based on the deterioration determination output VAFH using the following equation. (Step 12
a). LVAFH = LVAFH + | VAFH−VAFHO | Here, VAFHO is a deterioration determination output at the time of the previous execution.

Next, the locus length LVOS of the output of the O ₂ sensor 46 is determined using the following equation (step 12b). LVOS = LVOS + | VOS−VOSO | where VOSO is the output of the O ₂ sensor 46 at the time of the previous execution. Then, after the current deterioration determination output VAFH and the downstream O ₂ sensor output VOS are replaced with VAFHO and VOSO, respectively, in preparation for the next execution (step 12).
c), this process ends.

FIG. 15 is a flowchart of the deterioration determination process executed in step 1214. The counter CTIME for measuring the monitoring time is incremented (step 14a), and the counter CTIME is set to a predetermined value C _0.
Is determined (step 14b). Steps 14c to 14i when the predetermined monitor time has elapsed
Is performed.

That is, first, the output locus length L of the upstream A / F sensor
Calculating a threshold value L _ref for determining the deterioration of the three-way catalyst based on VAFH (step 14c). L _ref = or L _ref (LVAFH) then the O ₂ sensor output trajectory length LVOS is above the threshold value L _ref, i.e. to determine whether the three-way catalyst has deteriorated (step 14d).

If it is determined that the three-way catalyst has deteriorated, the alarm flag ALMCC is set to "1" (step 14e) and the alarm is activated (step 14).
f) and proceed to step 14h. Conversely, if it is determined that the three-way catalyst has not deteriorated, the alarm flag ALMC
C is set to "0" (step 14g), and step 1
Proceed to 4h.

In step 14h, the alarm flag A
The LMCC is stored in the B-RAM 79. This is a procedure for reading out the determination result at the time of repair and inspection. Further preparation for the next process, resets the monitoring time counter CTIME, the upstream A / F sensor output trajectory length LVAFH and O ₂ sensor output locus length LVOS (Step 14
i) This process ends.

If the predetermined monitor time has not elapsed, this processing is directly terminated. Although the case where the air-fuel ratio sensor installed downstream of the three-way catalyst is an O ₂ sensor has been described above, the present invention relates to a case where the air-fuel ratio sensor installed downstream of the three-way catalyst is an A / F sensor. Is also applicable. Since the processing in this case is almost the same as the above, only the differences will be described.

FIG. 16 is an explanatory diagram in the case where an A / F sensor is used as the upstream and downstream air-fuel ratio sensors. The horizontal axis represents time, and the upper part shows the upstream A / F sensor 45.
Output VAF (solid line) and deterioration determination output VAFH
(Broken line), and the lower row shows the output VA of the downstream A / F sensor 46.
FR (solid line) and its average value VAFRSM (dashed line)
Represents

That is, before time t _1, the downstream A / F sensor 4
The average value VAFRSM of 6 is rich, and while the lean spike of the air-fuel ratio of the gas containing the catalyst appears almost as it is, a small rich spike does not appear. Downstream A / F sensor 46 of a better value VAFRSM at time t ₁ becomes the stoichiometric air-fuel ratio, a lean spike and rich spike of the air-fuel ratio of the catalyst entering gas is almost appears as it is to the deterioration determining output VAFH.

After time t _{2, the} smoothed value VAFRSM of the downstream A / F sensor 46 is lean, and the rich spike in the air-fuel ratio of the gas containing the catalyst appears almost as it is, while the small lean spike is removed by the conversion function. Will be done. Therefore, the conversion function needs to be determined so as to compensate for the above characteristics. FIG. 17 is a second conversion map of the present embodiment, in which the horizontal axis represents the output VAF of the upstream air-fuel ratio sensor,
The vertical axis represents the conversion VAFH for deterioration determination.

That is, the smoothed value V of the downstream A / F sensor 46
When AFRSM is the stoichiometric air-fuel ratio (b), the conversion VAFH for deterioration determination is almost proportional to the upstream-side air-fuel ratio sensor output VAF. However, the average value V of the downstream A / F sensor 46
When the AFRSM is lean (a), the conversion value is set to "0" for a small change on the rich side, and when the smoothed value VAFRSM of the downstream A / F sensor 46 is rich (c), the conversion side is lean. For small changes in
0 ".

Therefore, conversion may be performed in step 1204 of the deterioration determination routine (FIG. 12) using the map shown in FIG. Further, when the downstream air-fuel ratio sensor is an A / F sensor, there is no possibility of erroneous determination due to the saturation characteristic of the sensor output, so that steps 1206 and 1208 in the deterioration determination main routine (FIG. 12) need not be executed.

[0064]

According to the catalyst deterioration judging device for an internal combustion engine according to the first , third and fourth to sixth aspects, the upstream side air space is calculated using a function corresponding to the variation of the equilibrium point of the oxygen balance in the three-way catalyst. By converting the output of the fuel ratio sensor to the output for determining the deterioration, it is possible to maintain the accuracy of determining the deterioration of the three-way catalyst regardless of the fluctuation of the equilibrium point of the oxygen balance in the three-way catalyst.

According to the catalyst deterioration determining apparatus for an internal combustion engine according to the second aspect, it is possible to know the fluctuation of the equilibrium point of the oxygen balance in the three-way catalyst by the moving average value of the output of the downstream air-fuel ratio sensor. According to the catalyst deterioration judging device for an internal combustion engine according to the seventh and eighth aspects, when the output of the upstream A / F sensor exceeds the upper and lower limit values, the deterioration judgment is stopped, so that the output of the upstream A / F sensor is reduced. Can be prevented from being erroneously determined even when the amplitude of.

[Brief description of the drawings]

FIG. 1 is a basic configuration diagram of a catalyst deterioration determination device for an internal combustion engine according to the present invention.

FIG. 2 is a detection characteristic diagram of an O ₂ sensor.

FIG. 3 is a detection characteristic diagram of an A / F sensor.

FIG. 4 is an explanatory diagram of a problem.

FIG. 5 is a configuration diagram of an embodiment of a catalyst deterioration determination device for an internal combustion engine according to the present invention.

FIG. 6 is a configuration diagram of an ECU.

FIG. 7 is a flowchart of a target in-cylinder fuel amount calculation routine.

FIG. 8 is an explanatory diagram of a storage state in a RAM.

FIG. 9 is a flowchart of a main air-fuel ratio feedback control routine.

FIG. 10 is a flowchart of a sub air-fuel ratio feedback control routine.

FIG. 11 is a flowchart of a fuel injection control routine.

FIG. 12 is a flowchart of a deterioration determination main routine.

FIG. 13 is a first conversion map.

FIG. 14 is a flowchart of a trajectory length calculation process.

FIG. 15 is a flowchart of a deterioration determination process.

FIG. 16 is an explanatory diagram in the case where the upstream and downstream air-fuel ratio sensors are A / F sensors.

FIG. 17 is a second conversion map.

[Explanation of symbols]

Reference Signs List 20 internal combustion engine 41 vacuum sensor 38 catalytic converter 45 upstream A / F sensor 46 downstream O ₂ sensor 50 reference position detection sensor 51 crank angle detection sensor 64 distributor 90 ECU

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) F01N 3/08-3/38 F01N 9/00-11/00 F02D 41/00-41/40 F02D 43 / 00-45/00

Claims (8)

(57) [Claims] An upstream air-fuel ratio sensor provided upstream of an exhaust purification catalyst provided in an exhaust passage of an internal combustion engine and having an output characteristic substantially proportional to an oxygen concentration in exhaust gas; Air-fuel ratio feedback control means for performing feedback control so that the engine air-fuel ratio becomes the target air-fuel ratio in accordance with the output of the sensor; provided on the downstream side of the exhaust gas purification catalyst, and having an output characteristic corresponding to the oxygen concentration in the exhaust gas. A downstream air-fuel ratio sensor, an air-fuel ratio center value estimating means for estimating an air-fuel ratio center value of the exhaust purification catalyst based on an output of the downstream air-fuel ratio sensor, and an air-fuel ratio center value estimating means. An upstream air-fuel ratio sensor output conversion means for converting the output of the upstream air-fuel ratio sensor into a deterioration determination output according to the air-fuel ratio center value in the exhaust purification catalyst; and the air-fuel ratio feedback. Control means for determining the deterioration of the exhaust gas purification catalyst based on the output for the deterioration determination and the output of the downstream air-fuel ratio sensor converted by the upstream air-fuel ratio sensor output conversion means within a predetermined period during the execution of the feedback control. And a catalyst deterioration determining means for determining whether the catalyst has deteriorated. 2. The catalyst deterioration judging device for an internal combustion engine according to claim 1, wherein the air-fuel ratio center value estimating means calculates a temporal moving average value of the output of the downstream air-fuel ratio sensor. 3. The upstream air-fuel ratio sensor output conversion means.
However, when the amplitude of the output of the upstream air-fuel ratio sensor increases,
Using the conversion characteristic that the output for
The output of the air-fuel ratio sensor is converted into the output for deterioration determination.
3. A device for determining catalyst deterioration of an internal combustion engine according to claim 1 or 2.
Place . 4. An upstream air-fuel ratio sensor output conversion means.
However, in the air-fuel ratio center value estimating means,
If it is determined that the air-fuel ratio center value is on the rich side,
Poor when the output of the upstream air-fuel ratio sensor is rich.
Uses conversion characteristics to correct the output for discrimination to the stoichiometric air-fuel ratio
To output the output of the upstream air-fuel ratio sensor to a deterioration determination output.
The 請 Motomeko 1 or 2 comprising a rich-side conversion means for converting
An apparatus for determining catalyst deterioration of an internal combustion engine according to claim 1 . 5. The upstream air-fuel ratio sensor output conversion means.
However, in the air-fuel ratio center value estimating means,
If it is determined that the air-fuel ratio center value is on the lean side,
Poor when the output of the upstream air-fuel ratio sensor is lean.
Uses conversion characteristics to correct the output for discrimination to the stoichiometric air-fuel ratio
To output the output of the upstream air-fuel ratio sensor to a deterioration determination output.
3. The method according to claim 1, further comprising a lean-side conversion means for performing conversion.
An apparatus for determining catalyst deterioration of an internal combustion engine according to claim 1 . 6. An upstream air-fuel ratio sensor output conversion means.
However, in the air-fuel ratio center value estimating means,
If it is determined that the air-fuel ratio center value is on the rich side,
Poor when the output of the upstream air-fuel ratio sensor is rich.
Uses conversion characteristics to correct the output for discrimination to the stoichiometric air-fuel ratio
To output the output of the upstream air-fuel ratio sensor to a deterioration determination output.
Rich side conversion means for converting, and the air-fuel ratio center value estimation means,
If it is determined that the air-fuel ratio center value is on the lean side,
Poor when the output of the upstream air-fuel ratio sensor is lean.
Uses conversion characteristics to correct the output for discrimination to the stoichiometric air-fuel ratio
To output the output of the upstream air-fuel ratio sensor to a deterioration determination output.
3. The method according to claim 1, further comprising a lean-side conversion means for performing conversion.
An apparatus for determining catalyst deterioration of an internal combustion engine according to claim 1 . 7. The air-fuel ratio sensor according to claim 1, wherein the downstream air-fuel ratio sensor is an oxygen concentration sensor, and the upper air-fuel ratio sensor output based on the air-fuel ratio center value in the exhaust purification catalyst estimated by the air-fuel ratio center value estimating means. Upper and lower limit value determining means for determining a lower limit value, and when the output of the upstream air-fuel ratio sensor exceeds the upper and lower limit value determined by the upper and lower limit value determining means, the determination of deterioration of the exhaust purification catalyst by the deterioration determining means is stopped. 2. The catalyst deterioration determination device for an internal combustion engine according to claim 1, further comprising a deterioration determination stop unit that performs the determination. 8. The apparatus according to claim 7, wherein said upper / lower limit value determining means is provided with said upstream-side empty space.
Deterioration determination output converted by the fuel ratio sensor output conversion means
Although the power does not saturate, the output of the upstream air-fuel ratio sensor becomes saturated.
6. The internal combustion engine according to claim 5, wherein a value at which summing is started is set as an upper and lower limit value.
Seki's catalyst deterioration determination device.